1. Field of the Invention
The present invention relates to an electrical discharge machine, or the like, which cuts a workpiece through proper control of a gap between the workpiece and an electrode. In particular, the present invention precisely controls the average machining gap voltage between an electrode and a workpiece.
2. Description of the Related Art
FIG. 39 is a circuit diagram of a conventional electrical discharge machine. Electrode 1 is connected to the positive pole of diode 4, and the negative pole of diode 4 is connected to the negative pole of first direct-current power supply 3. The positive pole of the first direct-current power supply 3 is connected to one end of current limiting resistor 5, and the other end of the current limiting resistor 5 is connected to the positive pole of semiconductor switch 6. The negative pole of semiconductor switch 6 is connected to the workpiece 2. Output terminal 7a of oscillator circuit 7 is connected to control terminal 6a of semiconductor switch 6 so as to control operation of semiconductor switch 6.
FIG. 40 illustrates detailed oscillator circuit 7, which consists of clock pulse generator 4001, two-input OR circuits 4002-4004, flip-flop 4007, on-time counter 4008, off-time counter 4009, on-time setter 4010 for setting the length of time when output terminal signal 7a of oscillator circuit 7 is set to "1," and off-time setter 4011 for setting the length of time when output terminal 7a of oscillator circuit 7 is set to "0". On-time setter 4010 has output terminals identical in number to the bits of on-time counter 4008, and off-time setter 4011 has output terminals identical in number to the bits of off-time counter 4009. First match comparator circuit 4012 has two sets of compared data input terminals identical in number to the bits of on-time counter 4008. The output terminals of on-time setter 4010 are connected to the corresponding compared data input terminals on one side of first match comparator circuit 4012, and the output terminals of on-time counter 4008 are connected to the corresponding compared data input terminals on the other side thereof.
First match comparator circuit 4012 compares on-time data set in on-time setter 4010 and the value of the on-time counter 4008, and outputs "1" to output terminal 4012a when they match, and outputs "1" to output terminal 4012b when the value of on-time counter 4008 reaches a predetermined set value.
Second match comparator circuit 4013 has two sets of compared data input terminals identical in number to the bits of off-counter 4009. The output terminals of off-time setter 4011 are connected to the corresponding compared data input terminals on one side of match comparator circuit 4013, and the output terminals of off-time counter 4009 are connected to the corresponding compared data input terminals on the other side thereof. Second match comparator circuit 4013 outputs "1" to output terminal 4013a when off-time data set in off-time setter 4011 matches the value of off-time counter 4009.
N-output terminal 4007a of flip-flop 4007 is connected to one input terminal of two-input AND circuit 4005 and also to output terminal 7a of oscillator circuit 7. The other input terminal of two-input AND circuit 4005 is connected to output terminal 4001a of clock pulse generator 4001. Further, the output terminal of two-input AND circuit 4005 is connected to count input terminal 4008a of on-time counter 4008.
C-output terminal 4007b of flip-flop 4007 is connected to one input terminal of two-input AND circuit 4006 and the other input terminal of two-input AND circuit 4006 is connected to output terminal 4001a of clock pulse generator 4001. The output terminal of two-input AND circuit 4006 is connected to count input terminal 4009a of off-time counter 4009.
Output terminal 4012a of match comparator circuit 4012 is connected to one input terminal of two-input OR circuit 4003 and also to one input terminal of two-input OR circuit 4002. Further, the output terminal of two-input OR circuit 4002 is connected to R-input terminal 4007c of flip-flop 4007 and the output terminal of two-input OR circuit 4003 is connected to reset input terminal 4008b of on-time counter 4008.
The output terminal 4013a of match comparator circuit 4013 is connected to one input terminal of two-input OR circuit 4004 and also to S-input terminal 4007d of flip-flop 4007. Further, the output terminal of two-input OR circuit 4004 is connected to reset input terminal 4009b of off-time counter 4013.
Reset terminal 7d of oscillator circuit 7 is connected to the other input terminal of two-input OR circuit 4002, the other input terminal of two-input OR circuit 4003, and the other input terminal of two-input OR circuit 4004. Further, output terminal 4012b of match comparator circuit 4012 is connected to output terminal 7b of oscillator circuit 7.
The operation of oscillator circuit 7 illustrated in FIG. 40 will now be described.
First, assume that flip-flop 4007 has been reset, C-output terminal 4007b signal of the flip-flop 4007 is "1", and N-output terminal 4007a signal is "0". Since a clock pulse of a predetermined cycle is always output to output terminal 4001a of clock pulse generator 4001, the clock pulse is input to count input terminal 4009a of on-time counter 4009 via two-input AND circuit 4006. Every time the clock pulse is input, off-time counter 4009 counts up. When the value of off-time counter 4009 has become equal to that stored in off-time setter 4011, i.e. the off-time data, output terminal 4013a of match comparator circuit 4013 is set to "1", causing reset input terminal 4009b of off-time counter 4009 to be set to "1" via two-input OR circuit 4004, thereby resetting off-time counter 4009. At the same time, flip-flop 4007 is set.
When flip-flop 4007 is set, C-output terminal 4007b of the flip-flop 4007 is set to "0" and the N-output terminal 4007a is set to "1". Accordingly, the clock pulse is not output to the output terminal of two-input AND circuit 4006 and off-time counter 4009 stops counting. However, the clock pulse is now output to the output terminal of two-input AND circuit 4005 and on-time counter 4008 starts counting each pulse.
When the value of the on-time counter 4008 has reached a predetermined set value, a pulse is output to output terminal 4012b of match comparator circuit 4012. This pulse is then output to the output terminal 7b of oscillator circuit 7. On-time counter 4008 further continues counting, and when its value has become equal to the on-time data stored in on-time setter 4010, output terminal 4012a of match comparator circuit 4012 is set to "1". Accordingly, reset input terminal 4008b of on-time counter 4008 is set to "1" via two-input OR circuit 4003, causing the on-time counter 4008 to be reset. Also, R-input terminal 4007c signal of flip-flop 4007 is set to "1" via two-input OR circuit 4002, thereby resetting flip-flop 4007. Hence, on-time counter 4008, off-time counter 4009 and flip-flop 4007 are all reset, to the above-mentioned initial conditions.
Since the aforementioned operation is repeated cyclically, a pulse which is "1" during the on-time counting of on-time setter 4010 and is "0" during off-time counting of off-time setter 4011 is output to the output terminal 7a of the oscillator circuit 7. At a predetermined period of time after output terminal 7a has changed to "1", the pulse is output to output terminal 7b.
Flip-flop 4007 is reset by sending a reset pulse to reset terminal 7d of oscillator circuit 7 after flip-flop 4007 has been set. C-output terminal 4007b of flip-flop 4007 is connected to output terminal 7c of the oscillator circuit 7.
Referring to FIG. 39, diode 8 is connected at its negative pole to third direct-current power supply 9 having an output voltage E3. Third switch circuit 10, e.g., a semiconductor switch, is connected at its positive pole with the other pole of third direct-current power supply 9. The positive pole of diode 8 is connected to electrode 1. The negative pole of semiconductor switch 10 is connected to workpiece 2. One end of voltage dividing resistor 11 is connected to electrode 1, the other end thereof is connected to one end of voltage dividing resistor 12. The other end of the voltage dividing resistor 12 is connected to the workpiece 2. Input terminal 13a of differential amplifier 13 is connected to connection point 12a between voltage dividing resistors 11 and 12, and input terminal 13b of differential amplifier 13 is connected to workpiece 2. Output terminal 13c of differential amplifier 13 is connected to input terminal 14a of discharge detector circuit 14.
One input terminal 16a of two-input AND circuit 16 is connected to output terminal 7b of oscillator circuit 7 and the other input terminal 16b thereof is connected to output terminal 14b of discharge detector circuit 14. Output terminal 16c of two-input AND circuit 16 is connected to input terminal 15a of one-shot multivibrator 15. Output terminal 15b of one-shot multivibrator 15 is connected to control terminal 10a of semiconductor switch 10. Capacitor 17 extends across electrode 1 and workpiece 2 is indicated by dashed lines.
The operation of the circuit shown in FIG. 39 will now be described in reference to the operation timing chart in FIG. 41.
Oscillator circuit 7 oscillates at a predetermined cycle Ta and outputs a voltage having a waveform shown in FIG. 41b, to output terminal 7a. This voltage is a pulse which is "1" for a period of Tb and "0" for a period of Tc. The pulse is applied to control terminal 6a of semiconductor 6 so as to turn semiconductor switch 6 on for a period Tb and off for a period Tc. When semiconductor switch 6 is on, voltage E1 of first direct-current power supply 3 is applied to a gap between the electrode 1 and workpiece 2 hereinafter referred to as the "machining gap" via current limiting resistor 5 and diode 4 to start an electrical discharge.
The waveform shown in FIG. 41a indicates a voltage across the machining gap hereinafter referred to as "machining gap voltage" Eg. From a time when semiconductor switch 6 is turned on to when the discharge is started, the machining gap voltage Eg is equivalent to the voltage of the first direct-current power supply 3. Subsequently, machining gap voltage Eg reduces as the discharge begins, and eventually is equal to predetermined voltage Vg.
Since a voltage proportional to machining gap voltage Eg develops across voltage dividing resistor 12, the output voltage of the differential amplifier 13 is also proportional to machining gap voltage Eg. This voltage is input to the input terminal 14a of discharge detector circuit 14. In accordance with this voltage, discharge detector circuit 14 determines whether or not the machining gap voltage Eg is between first preset voltage ES1 and second preset voltage ES2. If the machining gap voltage Eg is between first preset voltage ES1 and second preset voltage ES2, the discharge detector circuit 14 determines that a discharge has occurred and outputs a "1" at output terminal 14b. If machining gap voltage Eg is not between first preset voltage ES1 and second preset voltage ES2, the discharge detector circuit 14 determines that a discharge has not occurred and outputs a "0" at output terminal 14b.
FIG. 41d shows the voltage waveform of output terminal 14b and FIG. 41c shows the waveform of voltage output to output terminal 7b of the oscillator circuit 7. The voltage waveform of output terminal 7b is a pulse waveform which rises a predetermined period of time Td after the rise of output terminal 7a of oscillator circuit 7 and falls in a predetermined length of time Te.
FIG. 41e illustrates the waveform of a voltage output to output terminal 16c of two-input AND circuit 16 which results from the input of output terminal 7b of oscillator circuit 7 shown in FIG. 41c and output terminal 14b of discharge detector circuit 14 shown in FIG. 41d into the corresponding input terminals of two-input AND circuit 16.
The voltage signal shown in FIG. 41e is input to input terminal 15a of one-shot multivibrator 15. A voltage signal shown in FIG. 41f which is set to "1" on the leading edge of the voltage in FIG. 41e and returns to "0" in predetermined time Ton is output to output terminal 15b of the one-shot multivibrator 15. The voltage signal in FIG. 41f is input to control terminal 10a of the semiconductor switch 10 and turns semiconductor switch 10 on when the voltage in FIG. 41f is "1". When semiconductor switch 10 is turned on, third direct-current power supply 9 is connected to the already discharging machining gap via semiconductor switch 10 and diode 8. At this time, third direct-current power supply 9 causes a discharge current to flow in the machining gap.
FIG. 41g shows the waveform of a current which flows in the machining gap. This current waveform is shaped like a saw tooth wave which increases at a predetermined slope, when either of semiconductor switches 6 or 10 are on, and falls when both semiconductor switches 6 and 10 are turned off. The current changes at a predetermined slope without changing sharply as shown in FIG. 41g because there is an inherent inductance (not illustrated) in the circuit. Output voltage E3 of third direct-current power supply 9 is ordinarily higher than the output voltage E1 of the first direct-current power supply 3 so that a larger current may flow when semiconductor switch 10 is on.
If, for example, the discharge is stopped for some reason while semiconductor switch 10 is on, output voltage E3 of third direct-current power supply 9 is applied to the machining gap, raising machining gap voltage Eg. In addition, even after the semiconductor switches 6 and 10 are turned off, stray capacity 17, having a typical magnitude of several thousand to ten thousand PF, causes machining gap voltage Eg to remain high.
FIG. 42 is an operation timing chart where the discharge has been stopped when semiconductor switch 10 is on. FIGS. 42a to 42g indicate voltage and current waveforms measured at the same points as in FIGS. 41a to 41g respectively. In FIG. 42, T.sub.0 indicates a point when semiconductor switch 10 is turned on. T.sub.1 indicates a point when the discharge has stopped while semiconductor switch 10 is on, and T.sub.2 a point when semiconductor switch 10 turns off after the point T.sub.1. T.sub.3 indicates a point when output terminal 7a signal of oscillator circuit 7 is set to "1" again and semiconductor switch 6 turns on after the point T.sub.2. T.sub.4 indicates a point when semiconductor switch 10, having turned on at the point T.sub.3, turns off. In FIG. 42, the waveforms in FIGS. 42a to 42f prior to the point T.sub.1 are similar to those shown in FIG. 41.
When a discharge stop occurs at point T.sub.1, the current shown in FIG. 42g quickly drops to zero and machining gap voltage Eg shown in FIG. 42a rises to output voltage E.sub.3 of third direct-current power supply 9. Also, the voltage shown in FIG. 42f automatically returns to zero at point T.sub.2 a predetermined length of time Ton after rising, as shown in FIG. 41f. From point T.sub.2 to point T.sub.3, the voltage in FIG. 42a is kept as high as output voltage E.sub.3 of third direct-current power supply 9 by stray capacitance 17.
At point T.sub.3, output terminal 7a of oscillator circuit 7 is set to "1" and semiconductor switch 6 is turned on to begin the next discharge operation. The waveforms indicating the operation after point T.sub.3 are similar to those shown in FIG. 41 which correspond to a normal discharge operation.
As illustrated in FIG. 42a, when a high voltage is applied to the machining gap because of a discharge termination occurring while semiconductor switch 10 is on, the average voltage in the machining gap quickly rises.
Meanwhile, a positive average voltage applied to the machining gap (hereinafter referred to as the "average machining voltage") is kept constant to maintain the machining gap conditions required for electrical discharge machining. Namely, when the average machining voltage is higher than a predetermined value, the machining gap is decreased to reduce the average machining voltage. When it is lower than the predetermined value, the machining gap is increased to raise the average machining voltage. This is achieved by moving a table holding workpiece 2 or an electrode support holding electrode 1 during machining.
Since a positive voltage is only applied to the machining gap in the discharge circuit shown in FIG. 39, the aforementioned average machining gap voltage is identical to the above described average machining voltage.
When a high voltage develops in the machining gap due to a discharge stop while semiconductor switch 10 is on, the average machining voltage rises to at least a voltage equivalent to the machining gap voltage, causing the control for maintaining the machining gap constant to be faulty. That is, the machining gap decreases abnormally and a centralized discharge is generated. Such a centralized discharge can damage electrode 1 and workpiece 2 and reduce machining accuracy.
Another method of maintaining the machining gap constant, without using the average machining voltage, may be carried out according to the length of no-load time between the turning on of semiconductor switch 6 and a discharge start. This method, however, cannot be employed for an electrical discharge machine which has a large stray capacity 17 and whose dielectric is relatively conductive. This is so because current limiting resistor 5 used in such a machine requires a long time from when semiconductor switch 6 is turned on to when machining gap voltage Eg rises, and machining gap voltage Eg falls below voltage E.sub.1.
Unlike the discharge circuit of the conventional electrical discharge machine shown in FIG. 39 employing two power supplies, some electrical discharge machines known in the art have only one power supply. FIG. 43 is a discharge circuit diagram of such an electrical discharge machine.
Electrode 1 is connected to the negative pole of first direct-current power supply 4301, the positive pole of first direct-current power supply 4301 is connected to one end of current limiting resistor 4303, the other end of the current limiting resistor 4303 is connected to the positive pole of semiconductor switch 4302, and the negative pole of semiconductor switch 4302 is connected to workpiece 2. Output terminal 7a of the oscillator circuit 7 is connected to control terminal 4302a of semiconductor switch 4302.
The operation of the discharge circuit illustrated in FIG. 43 will be described with reference to the operation timing chart shown in FIG. 44.
A pulse, alternately repeating over a period T44b when the output is "1" and a period T44c when the output is "0" as shown in FIG. 44b, is output to output terminal 7a of the oscillator circuit 7. This pulse is input to control terminal 4302a of semiconductor switch 4302 to keep semiconductor switch 4302 on during period T44b and off during period T44c.
FIG. 44a shows the waveform of machining gap voltage Eg which rises to voltage E3510 on a leading edge T4400 of the waveform in FIG. 44b, falls to a voltage Vg1 at a discharge start point T4401, and finally falls to zero on trailing edge T4402 of the waveform in FIG. 44b.
FIG. 44c shows the waveform of discharge current. In this waveform, the current increases at a predetermined slope, starting at point T4401 and decreases to zero at sharp slope, starting at point T4402.
While the operation timing chart shown in FIG. 44 illustrates an ordinary operating state, FIG. 45 is an operation timing chart illustrating an operation where a discharge stop has taken place for some reason at a point T4501 between points T4401 and T4402 and the discharge is not resumed even at point T4402.
Machining gap voltage Eg is held at almost the voltage E3510 between point T4402 and point T4403 when output terminal 7a of oscillator circuit 7 is set to "1" again. Such a high voltage is maintained because electrical charges are accumulated due to inherent stray capacitance 4305 of the machining gap. Since machining gap voltage Eg is high between points T4402 and T4403, the average machining voltage is different from that in the case where the no-load time occurs once per predetermined cycle as shown in FIG. 44. Thus, normal machining gap control is not carried out.
FIG. 46 shows a case where the discharge is resumed at point T4601 before point T4402 after a discharge stop has taken place at point T4501 in FIG. 45. Since the no-load time is between points T4501 and T4601 in this case, the no-load time of high machining gap voltage Eg occurs twice in one cycle. Hence, the average machining voltage rises abnormally and machining gap control is not conducted normally in this case, either.
In the conventional electrical discharge machine constructed as described above, if a discharge stops for some reason, a high machining voltage occurs in the machining gap and the average machining voltage rises abnormally. At this time, the machining gap controlled in accordance with the average machining voltage decreases abnormally, generating a centralized discharge, possibly damaging electrode 1 and workpiece 2 and reducing machining accuracy.